Clearly he is in love with someone and is trying to do as much for them as possible to show them this ("When the world gets too heavy put it on my back, I'll be your levy"), but that person is either disregarding his kindness or hurting him in return ("You are taking me apart like bad glue on a get well card"). The 'get well card' is a symbol of his affection and what he is doing for his love, and the 'bad glue' symbolises the hurt directed towards him from that person which is, as he says, "taking me apart".

I get the feeling that this was someone that used to like/be in love with him ("It was always you falling for me") and now he's desperately trying to get them back.

I cannot for the life of me figure out what "Now there's always time calling for me" means. Perhaps it means that he is prepared to wait forever for this person.

He's trying desperately to show this person that he loves them, and is searching desperately for a sign that they feel the same way ("I'm the light blinking at the end of the road, blink back to let me know"). The 'light' meaning that he believes he is best for this person and the 'blinking' is a metaphor for letting the person know how he feels, and his hope that they will 'blink back'.

He is trapped in his love for this person ("I'm a fly that's trapped in a web") but he believes that he may be trapped forever ("But I'm thinking that my spider's dead" - the spider being his way out; the spider is this other person admitting that they feel the same way).

He is obviously lonely, and this has made him depressed, though he puts on a mask to appear fine to those around him ("Lonely, lonely little life, I could kid myself thinking that I'm fine").

This whole situation has worn him down completely ("That I'm skin and bone" - meaning that there is nothing left of him but skin and bone; he has sacrificed a lot for this person to try to make them happy, at a great cost (emotionally and mentally) to himself).

He used to be a strong person, but cannot handle it any longer. (Just a king and a rusty throne oh, the castle's under siege - 'king' symbolising his strength, and the 'rusty throne' symbolising the deterioration of this strength. The castle could be his mind, which is under attack, which could then be a metaphor for his mental suffering.)

He wants to deal with things himself, and doesn't want help from anyone else ("But the sign outside says 'leave me alone'").